1. Field of the Invention
The present invention generally relates to the art of compressing a fluid in an oil-injected rotary screw compressor. More specifically, the present invention relates to a variable oil flow regulator for a rotary compressor for minimizing noise generated by the compressor when the compressor transitions from an on-load mode to an off-load mode.
2. Brief Description of the Prior Art
Rotary compressors used to compress fluids, such as air or a refrigerant, generally include intermeshed male and female screw rotors mounted within a compression chamber of a rotor housing. The compression chamber may include a pair of parallel intersecting cylindrical bores which are sized to match the outside length and diameter dimensions of the intermeshed screw rotors. The intermeshed screw rotors are generally paired with one another so that a male rotor is paired with a female rotor, however, certain compressors may include three or more inter-meshed rotors.
During operation of the compressor, fluid (i.e., ambient air) at suction pressure enters the compression chamber via an inlet and is enveloped in a chevron-shaped pocket formed between the rotating screw rotors. As the rotors mesh and rotate within the compression chamber, the chevron-shaped pocket shrinks for decreasing the volume of the pocket and the pocket is displaced toward the outlet or high pressure end of the compressor. The air within the pocket is compressed by virtue of the decreasing volume of the pocket during rotation of the rotors. Due to the tight fit between the rotors and the walls of the intersecting cylindrical bores, the bearing arrangement in which the rotors are mounted is critical to efficient operation and life-span of the compressor. In addition, proper lubrication of the compression chamber with oil is of paramount concern in the design and operation of rotary screw compressors.
Oil is typically injected into the compression chamber through an injection port. The oil performs many useful functions. First, the injected oil provides a sealant between the intermeshed rotors and the cylindrical bores. The oil also acts as a lubricant between the driving screw rotor (i.e., a male rotor) and the driven screw rotor (i.e., a female rotor). As is known to those skilled in the art, one of the two screw rotors is generally driven by an external source, such as an electric motor, while the other rotor is driven by virtue of its meshing relationship with the motor-driven rotor. Oil injected into the compression chamber of the compressor therefore acts to prevent excessive wear between the driving and driven rotors. Finally, injected oil is used to cool the fluid undergoing compression within the compression chamber. This cooling methodology reduces thermal expansion of the rotors that would otherwise occur as a result of the heat generated during the compression process. In addition, utilizing the oil as a coolant permits even tighter clearances between the rotors and the cylindrical bores within the rotor housing.
Due to the important role which oil may play during the compression process, the proper control of the volume and flow of oil throughout the compressor is critical. One particular method of control uses an oil stop valve for stopping the flow of oil when the compressor shuts down, thereby preventing overflow and the possibility of hydraulic lock upon a restart. The oil stop valve is frequently controlled electrically, by means of a solenoid valve which is generally costly and complicated. A simplified oil control valve is disclosed in commonly assigned U.S. Pat. No. 4,639,196 to Kirkland which teaches a fluid control valve for controlling oil flow in and to a compressor which requires no electrical controls, and no spring biasing, and which comprises a simple, cartridge-type housing to facilitate its employment in a compressor housing.
Although the '196 patent has provided an effective and simple oil control valve for stopping the flow of oil during a compressor shutdown, there are many instances when it is undesirable to continuously stop and restart a compressor in response to demand. For example, certain compressors utilize star-delta motors which may only be started a limited number of times within a set time frame. Specifically, one particular star-delta motor may only be started 20 times within a one hour period. Attempts to start this particular motor more than 20 times in one hour may result in severe and permanent damage to the motor. As such, instead of continuously stopping and starting the motor, in certain instances it may be preferable to continuously operate the compressor. With these continuously operating compressors, when there is a demand for compressed air, the compressor is placed in an on-load mode so that the compressor may produce compressed air. However, when demand ceases, the compressor transitions to an off-load mode whereby the inlet valve for the compressor is at least partially (and preferably completely) closed and the compressed fluid stored in a compressed fluid storage tank is vented to atmosphere.
When transitioning from the on-load mode to the off-load mode, it is well known to those skilled in the art that the rotary compressor typically creates transition noise. Transition noise is frequently described as a rattling or howling noise produced by the rotors when the compressor transitions from the on-load mode to the off-load mode. Although the present invention is not limited by any particular theory of operation, it is believed that the compressible fluid or air introduced into the compression chamber tends to keep the rotors away from one another. However, when the inlet valve is at least partially closed the rotors are not compressing a standard amount of air so that the rotors tend to become unstable and being to make howling or rattling noises. It is believed that the rattling and/or howling noises result from a rapid pressure change within the compression chamber, resulting in the rotors becoming unstable with respect to one another.
Thus, there is a need for a device for use with a rotary compressor or a method which minimizes transition noise when the compressor transitions from an on-load mode to an off-load mode. In particular, there is a need for a variable oil flow regulator which is capable of reducing the flow of the oil and compressible fluid introduced into the compression chamber during the transition period in order to minimize the transition noises traditionally generated within the compressor during this period.